The significant polarization components of a medium produced by contact with an electric field are first order polarization (linear polarization), second order polarization (first nonlinear polarization), and third order polarization (second nonlinear polarization). On a molecular level this can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 . . .( 1)
where
P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA1 .beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3. PA1 P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium. PA1 .chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.(3) E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3. . .( 2)
where
To achieve on a macromolecular level second order polarization, .chi..sup.(2) E.sup.2, of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
A significant difficulty encountered in finding materials exhibiting usefully large second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies). Materials having usefrully large values of .beta. are commonly referred to as molecular dipoles.
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are placed in an electric field.
Second order polarization, .chi..sup.(2) E.sup.2, has been suggested to be useful for a variety of purposes, including parametric effects, most notably frequency doubling, also referred to as second harmonic generation (SHG). Frequency doubling has attracted particular attention, since laser diodes are not readily constructed that can emit shorter wavelengths, but their outputs when doubled in frequency provide these wavelengths.
For a number of years the materials employed for achieving second order polarization effects were noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. David J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, postulated mathematically and experimentally corroborated second order polarizabilities in organic molecular dipoles equalling and exceeding those of inorganic crystals. Electrical poling and Langmuir-Blodgett construction techniques were recognized from the outset to be feasible approaches for polar alignment of the organic molecular dipoles to translate molecular second order polarizabilities into layer second order polarization susceptibilities. Zyss, "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, Vol. 1, pp. 25-45, 1985, is essentially cumulative with Williams, surveying applications for organic molecular dipoles to varied nonlinear optical needs.
Garito U.S. Pat. No. 4,431,263; Girling, Cade, Kolinsky, and Montgomery, "Observation of Second Harmonic Generation from a Langmuir-Blodgett Monolayer of a Merocyanine Dye," Electronics Letters, Vol. 21, No. 5, Feb. 28, 1985; Neal, Petty, Roberts, Ahmad, and Feast, "Second Harmonic Generation from LB Superlattices Containing two Active Components," Electronics Letters, Vol. 22, No. 9, Apr. 24, 1986; and Ulman et al U.S. Pat. No. 4,792,208 provide illustrations of organic molecular dipoles deposited by Langmuir-Blodgett techniques to form layers exhibiting significant .chi..sup.(2) values.
Williams and Zyss are extrapolations from limited demonstrated capabilities to theoretically possible applications, including second harmonic generation. Garito, Girling et al, Neal et al, and Ulman are concerned with Langmuir-Blodgett components to meet device requirements.
What has been absent from the art are optical device constructions that go beyond the bare minimum features for corroborating the theoretical feasibility of frequency doubling to structural features necessary for high conversion efficiencies. Akhemediev and Novak, Opt. Spectros. (USSR) 58 (4), 558 (1985) represents a first, albeit theoretical step in the direction of improving conversion efficiencies by mathematically modeling a Langmuir-Blodgett bilayer construction exhibiting .chi..sup.(2) values of opposite sign to counteract cancelling positive and negative amplitudes in the second harmonic electric field. There is no indication that Akhemediev et al actually built an optical article or had the capability of actually building the type of device construction mathematically modelled.
Electronic and Photonic Applications of Polymers, M. J. Bowden and S. R. Turner Ed., Chapter 6, Polymers in Nonlinear Optics, by D. Williams, American Chemical Society 1988, suggests in FIG. 6.18 a waveguide construction similar to that of Akhemediev et al using X or Z type LB assemblies.
Penner et al U.S. Ser. No. 07/735,551, filed Jul. 25, 1991, now abandoned, and commonly assigned, entitled HIGH SECOND ORDER POLARIZATION SUSCEPTIBILITY AND LOW TRANSMISSION ATTENUATION, discloses an optical article comprised of an organic layer unit exhibiting a second order polarization susceptibility greater than 10.sup.-9 esu and means for providing an optical input to and an optical output from the layer unit. The organic layer unit exhibits a transmission attenuation of less than 2 dB/cm and is comprised of a Y type Langmuir-Blodgett assembly having superimposed oriented monomolecular layers of first and second polymeric amphiphiles each containing repeating units comprised of a hydrophilic moiety and a lipophilic moiety. Repeating units of one or both of the first and second amphiphiles each contain an organic molecular dipole, and repeating units of one or both of the first and second amphiphiles each contains a branched lipophilic moiety of up to 9 carbon atoms.